A method and a device are known from WO 99/47702 A2. In this known method and this known device, the marking substance is brought into contact with a detector substance which has a specific property corresponding to the marking substance. The marking and detector substances are each configured as a nucleotide sequence, especially as a deoxyribonucleic acid (=DNA) or a peptide nucleic acid (=PNA). If the mutually matched nucleotide sequences are brought into contact with one another in a particular way, hybridization takes place. The nucleotide sequence of the detector substance is furthermore linked to at least one fluorophore molecule. Owing to measures which are additionally provided, this molecule does not develop its fluorescent property until after hybridization with the nucleotide sequence of the marking substance. By exposure to a light signal with a corresponding wavelength, a fluorescence reaction can then be induced which is employed to identify the marking.
WO 99/47702 A2 gives no indications about how the fluorescence reaction can be precisely detected. In fact, the received light radiation is made up of a first component, due to the fluorescence reaction of the hybridized marking, and a second component which results from reflection by the marking and, above all, by the marking background. Since only the fluorescent component of the marking identification can be used, it is necessary to separate this component. However, WO 99/47702 A2 provides no information about this. In the least favorable case, the two radiation components cannot be separated and no identification is possible.
A device, designed as a so-called fluorescence mirror scanner, for detecting an invisible marking which fluoresces in the near infrared wavelength is known from WO 97/50053 A2. In order to be able to separate the component coming from the background from the component caused by the fluorescence reaction in the received radiation, either tracking of the radiation source power or a special background material is provided in the disclosed device. The tracking is carried out so that the component caused by the background in the received radiation is kept constant. However, the power tracking entails a relatively complicated structure with a regulating unit. The other components used for constructing the fluorescence mirror scanner are also elaborate. The described device is furthermore not easy to transport and, in particular, it cannot be used as a hand-held instrument.
The same is true of another device for detecting an invisible fluorescent marking, which is described in U.S. Pat. No. 5,331,140. In order to separate the component caused by the fluorescence in the received radiation, the radiation used for excitation is modulated with two frequencies. An intermodulation component of these two frequencies is extracted from the electrical signal derived from the received radiation. The intermodulation product results from nonlinear behavior which is associated with the fluorescence reaction. The frequency modulation and separation of the intermodulation product entail outlay which is not inconsiderable. Other complicated components are furthermore used, for example the adjustable deflection mirror which directs the exciting radiation onto the various regions of the marking.
A device for identifying a marking which contains fluorescent material is described in JP 11-306276 A. The device is configured in the form of a hand-held instrument. An expensive optical filter is used for separating the background radiation and the fluorescent radiation in the received radiation. The device, just like all the others mentioned above, is furthermore suitable only for the detection of a marking which already fluoresces before the start of the measurement.